1. Field of the Invention
This invention relates to a photographic lens, and, more particularly, it is concerned with a rear conversion lens of a highly compact type, which is mounted on the photographic lens to the side of a photographic image, thereby extending the focal length of the lens system.
2. Description of the Prior Art
Various kinds of rear conversion lens (hereinafter referred to as "RCL") have been put into practical use. However, the RCL which is mountable on a photographic lens has been all limited to those having a long back focus. The RCL for a single lens reflex camera, when its magnification is made constant, varies its lens performance to a significant extent depending on the back focus length, the position of the exit pupil, and further the F-number of the objective lens. On account of this, changes in the standard of the objective lens to be mounted on the RCL affect the lens structure of the RCL.
FIG. 1 of the accompanying drawing illustrates a schematic construction, wherein an RCL 30 is interposed between an objective lens 10 and a body 20 of the single lens reflex camera. The illustration includes a marginal ray extending from an infinite object point on the optical axis and reaching a film surface 21. The body 20 of the single lens reflex camera contains therein an oscillatable reflection mirror 22, a reticle 23, a condenser lens 24, a pentagonal Dach (roof) prism 25, and an ocular lens 26. The reflection mirror 22 is usually at an oblique position as shown by a dotted line, when no exposure is to be performed on the film surface 21. In order to secure a space for oscillation of this oscillatable reflection mirror 22 in the single lens reflex camera, a distance between a lens mounting plane 28 and the film plane 21 in the camera body 20, i.e., the so-called "flange back (MB)" is predetermined to a fixed value. On the other hand, a distance between the surface of the rearmost lens in the objective lens and its focal point, i.e., a back focus (B.sub.f ') is designed to be sufficiently longer than the oscillating space for the reflection mirror 22. As the consequence of this, the back focus (B.sub.f) of a composite system comprising the objective lens and the RCL mounted on the objective lens should be maintained greater than the oscillating space for the reflecting mirror.
In the RCL having a magnification ranging from 1.4 to 1.5 or so, and mounted on a lens for a single lens reflex camera with a large aperture ratio, if the exit pupil of the objective lens, on which the RCL is mounted, is at a relatively distant position, the effective aperture of each lens constituting the RCL becomes inevitably large because the marginal ray reaching the end part of the image plane passes through a location away from the optical axis. From the standpoint of correcting the aberrations, an optical system, which is durable for use under a bright circumstance, is also required. Since the magnification of the RCL is only 1.4 to 1.5 or so, as mentioned above, when it is mounted on an objective lens having a short back focus, there is great difficulty in securing sufficient back focus of the lens for a single lens reflex camera after mounting of the RCL. With a view to solving these difficulties, there may be contemplated methods of reducing the center thickness of each lens component in the RCL, reducing an aerial interspace among the lens components, and decreasing the number of lens components for the lens structure to thereby reduce the total lens thickness of the RCL as far as possible. In so doing, it is possible to secure the sufficient back focus, but freedom in correction of the aberrations is sacrificed, with the consequent difficulties in obtaining a lens system having a large aperture ratio and a high lens performance.